Inserting virtual carrier in conventional OFDM host carrier in communications system

ABSTRACT

A base station communicating data to/from plural mobile terminals over plural OFDM sub-carriers within a coverage area. The base station allocates transmission resources provided by a first group of the plural OFDM sub-carriers within a first frequency band to mobile terminals of a first type and allocates transmission resources provided by a second group of the plural OFDM sub-carriers within a second frequency band to terminals of a second type, the second group being smaller than the first group and the second frequency band selected from within the first frequency band. The base station transmits control information including resource allocation information for terminals of the first type over a first bandwidth corresponding to the combined first and second groups of OFDM sub-carriers and transmits control information including resource allocation information for terminals of the second type over a second bandwidth corresponding to the second group of OFDM sub-carriers.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to British Patent Application1101970.0 filed in the U.K. on Feb. 4, 2011, the entire contents ofwhich are incorporated hereby reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to methods, systems and apparatus forallocating transmission resources and transmitting data in mobiletelecommunication systems.

BACKGROUND OF THE INVENTION

Third and fourth generation mobile telecommunication systems, such asthose based on the 3GPP defined UMTS and Long Term Evolution (LTE)architecture are able to support more sophisticated services than simplevoice and messaging services offered by previous generations of mobiletelecommunication systems.

For example, with the improved radio interface and enhanced data ratesprovided by LTE systems, a user is able to enjoy high data rateapplications such as mobile video streaming and mobile videoconferencing that would previously only have been available via a fixedline data connection. The demand to deploy third and fourth generationnetworks is therefore strong and the coverage area of these networks,i.e. geographic locations where access to the networks is possible, isexpected to increase rapidly.

The anticipated widespread deployment of third and fourth generationnetworks has led to the parallel development of a class of devices andapplications which, rather than taking advantage of the high data ratesavailable, instead take advantage of the robust radio interface andincreasing ubiquity of the coverage area. Examples include so-calledmachine type communication (MTC) applications, which are typified bysemi-autonomous or autonomous wireless communication devices (i.e. MTCdevices) communicating small amounts of data on a relatively infrequentbasis. Examples include so-called smart meters which, for example, arelocated in a customer's house and periodically transmit information backto a central MTC server data relating to the customers consumption of autility such as gas, water, electricity and so on.

Whilst it can be convenient for a terminal such as an MTC type terminalto take advantage of the wide coverage area provided by a third orfourth generation mobile telecommunication network there are at presentdisadvantages. Unlike a conventional third or fourth generation mobileterminal such as a smartphone, an MTC-type terminal is preferablyrelatively simple and inexpensive. The type of functions performed bythe MTC-type terminal (e.g. collecting and reporting back data) do notrequire particularly complex processing to perform. However, third andfourth generation mobile telecommunication networks typically employadvanced data modulation techniques on the radio interface which canrequire more complex and expensive radio transceivers to implement. Itis usually justified to include such complex transceivers in asmartphone as a smartphone will typically require a powerful processorto perform typical smartphone type functions. However, as indicatedabove, there is now a desire to use relatively inexpensive and lesscomplex devices to communicate using LTE type networks.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided abase station for communicating data to and from a plurality of mobileterminals over a plurality of OFDM sub-carriers within a coverage areaprovided by the base station. The base station arranged to allocatetransmission resources provided by a first group of the plurality ofOFDM sub-carriers within a first frequency band to mobile terminals of afirst type and to allocate transmission resources provided by a secondgroup of the plurality of OFDM sub-carriers within a second frequencyband to terminals of a second type, the second group being smaller thanthe first group and the second frequency band being selected from withinthe first frequency band. The base station is also arranged to transmitcontrol information comprising resource allocation information forterminals of the first type over a first bandwidth corresponding to thecombined first and second groups of OFDM sub-carriers and to transmitcontrol information comprising resource allocation information forterminals of the second type over a second bandwidth corresponding tothe second group of OFDM sub-carriers.

In conventional OFDM based mobile telecommunication networks, controldata is typically transmitted from the network to the mobile terminalsin a control channel which spans substantially the whole of thebandwidth of the signal transmitted from the base station. Normally amobile terminal cannot operate within the network unless this controldata is received and decoded and therefore the use of mobile terminalsthat operate with a bandwidth that is less than the whole bandwidth ofthe base station is precluded.

However, in accordance with this aspect of the invention, a subset ofthe OFDM sub-carriers are defined that are arranged across a reducedbandwidth. Data for reduced capability mobile terminals, includingcontrol data, can be separately transmitted on this subset of the OFDMsub-carriers.

This subset of the OFDM sub-carriers forms a “virtual carrier” within aconventional OFDM type downlink carrier (i.e. a “host carrier”). Unlikedata transmitted on a conventional OFDM type downlink carrier, datatransmitted on the virtual carrier can be received and decoded withoutneeding to process the full bandwidth of the downlink host OFDM carrier.Accordingly, data transmitted on the virtual carrier can be received anddecoded using a reduced complexity transceiver unit.

Devices provided with such a reduced complexity receiver unit (onwardsreferred to as “virtual carrier terminals”) can be constructed to beless complex and less expensive than conventional LTE type devices(onwards referred to generally as LTE terminals). Accordingly, thewidespread deployment of devices for MTC type applications within an LTEtype network which was previously impractical due to the prohibitivecost of conventional LTE terminals is now more practical because of thereduced cost of the virtual carrier terminals made possible by theprovision of the virtual carrier.

Furthermore, in some examples, the virtual carrier inserted within thehost carrier can be used to provide a logically distinct “network withina network”. In other words data being transmitted via the virtualcarrier can be treated as logically distinct from the data transmittedby the host carrier network. The virtual carrier can therefore be usedto provide a so-called dedicated messaging network (DMN) which is “laidover” a conventional network and used to communicate messaging data toDMN devices (i.e. virtual carrier terminals).

In one embodiment of the invention in which the second group of theplurality of the OFDM sub-carriers form a virtual carrier inserted inthe first group of the plurality of the OFDM sub-carriers and the firstgroup of the plurality of the OFDM sub-carriers form a host carrier, thebase station is arranged to transmit data to the terminals of the firsttype on the host carrier and transmitting data to or from the terminalsof the second type on the virtual carrier.

In accordance with this embodiment, the definition of a virtual carrierprovides a convenient mechanism by which data transmitted to terminalsof the second type (e.g. reduced capability terminals) can be logicallydistinguished from data transmitted to terminals of the first type (e.g.conventional terminals). In some examples multiple virtual carriers areprovided.

In accordance with one embodiment of the invention, the base station isarranged to transmit reference signals for use by both the terminals ofthe first type and terminals of the second type in the virtual carrier.In one example this comprises transmitting additional reference signalsfor use by terminals of the second type in the virtual carrier. Thisenables terminals of the second type (e.g. the reduced capabilityterminals) to improve the quality of the channel estimation which wouldotherwise be reduced in quality by virtue of the limited number ofreference signals transmitted in the virtual carrier.

Various further aspects and embodiments of the invention are provided inthe appended claims.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the present invention will now be described by way ofexample only with reference to the accompanying drawings where likeparts are provided with corresponding reference numerals and in which:

FIG. 1 provides a schematic diagram illustrating an example of aconventional mobile telecommunication network;

FIG. 2 provides a schematic diagram illustrating a conventional LTEradio frame;

FIG. 3 provides a schematic diagram illustrating an example of aconventional LTE downlink radio sub-frame;

FIG. 4 provides a schematic diagram illustrating a conventional LTE“camp-on” procedure;

FIG. 5 provides a schematic diagram illustrating an LTE downlink radiosub-frame in which a virtual carrier has been inserted in accordancewith an embodiment of the invention;

FIG. 6 provides a schematic diagram illustrating an adapted LTE“camp-on” procedure for camping on to a virtual carrier;

FIG. 7 provides a schematic diagram illustrating LTE downlink radiosub-frames in accordance with an embodiment of the present invention;

FIG. 8 provides a schematic diagram illustrating a physical broadcastchannel (PBCH);

FIG. 9 provides a schematic diagram illustrating an LTE downlink radiosub-frame in accordance with an embodiment of the present invention;

FIG. 10 provides a schematic diagram illustrating an LTE downlink radiosub-frame in which a virtual carrier has been inserted in accordancewith an embodiment of the invention;

FIGS. 11A to 11D provide schematic diagrams illustrating positioning oflocation signals within a LTE downlink sub-frame according toembodiments of the present invention;

FIG. 12 provides a schematic diagram illustrating a group of sub-framesin which two virtual carriers change location within a host carrier bandaccording to an embodiment of the present invention;

FIGS. 13A to 13C provide schematic diagrams illustrating LTE uplinksub-frames in which an uplink virtual carrier has been inserted inaccordance with an embodiment of the present invention, and

FIG. 14 provides a schematic diagram showing part of an adapted LTEmobile telecommunication network arranged in accordance with an exampleof the present invention.

DESCRIPTION OF EXAMPLE EMBODIMENTS

Conventional Network

FIG. 1 provides a schematic diagram illustrating the basic functionalityof a conventional mobile telecommunications network.

The network includes a plurality of base stations 101 connected to acore network 102. Each base station provides a coverage area 103 (i.e. acell) within which data can be communicated to and from mobile terminals104. Data is transmitted from a base station 101 to a mobile terminal104 within a coverage area 103 via a radio downlink. Data is transmittedfrom a mobile terminal 104 to a base station 101 via a radio uplink. Thecore network 102 routes data to and from the mobile terminals 104 andprovides functions such as authentication, mobility management, chargingand so on.

Mobile telecommunications systems such as those arranged in accordancewith the 3GPP defined Long Term Evolution (LTE) architecture use anorthogonal frequency division multiplex (OFDM) based interface for theradio downlink (so-called OFDMA) and the radio uplink (so-calledSC-FDMA). Data is transmitted on the uplink and on the downlink on aplurality of orthogonal sub-carriers. FIG. 2 shows a schematic diagramillustrating an OFDM based LTE downlink radio frame 201. The LTEdownlink radio frame is transmitted from an LTE base station (known asan enhanced Node B) and lasts 10 ms. The downlink radio frame comprisesten sub-frames, each sub-frame lasting 1 ms. A primary synchronisationsignal (PSS) and a secondary synchronisation signal (SSS) aretransmitted in the first and sixth sub-frames of the LTE frame. Aprimary broadcast channel (PBCH) is transmitted in the first sub-frameof the LTE frame. The PSS, SSS and PBCH are discussed in more detailbelow.

FIG. 3 provides a schematic diagram providing a grid which illustratesthe structure of an example of a conventional downlink LTE sub-frame.The sub-frame comprises a predetermined number of symbols which aretransmitted over a 1 ms period. Each symbol comprises a predeterminednumber of orthogonal sub-carriers distributed across the bandwidth ofthe downlink radio carrier.

The example sub-frame shown in FIG. 3 comprises 14 symbols and 1200sub-carriers spaced across a 20 MHz bandwidth. The smallest unit onwhich data can be transmitted in LTE is twelve sub-carriers transmittedover one sub-frame. For clarity, in FIG. 3, each individual resourceelement is not shown, instead each individual box in the sub-frame gridcorresponds to twelve sub-carriers transmitted on one symbol.

FIG. 3 shows resource allocations for four LTE terminals 340, 341, 342,343. For example, the resource allocation 342 for a first LTE terminal(UE 1) extends over five blocks of twelve sub-carriers, the resourceallocation 343 for a second LTE terminal (UE2) extends over six blocksof twelve sub-carriers and so on.

Control channel data is transmitted in a control region 300 of thesub-frame comprising the first n symbols of the sub-frame where n canvary between one and three symbols for channel bandwidths of 3 MHz orgreater and where n can vary between two and four symbols for channelbandwidths of 1.4 MHz. For clarity, the following description relates tohost carriers with channel bandwidth of 3 MHz or greater where themaximum value of n will be 3. The data transmitted in the control region300 includes data transmitted on the physical downlink control channel(PDCCH), the physical control format indicator channel (PCFICH) and thephysical HARQ indicator channel (PHICH).

The PDCCH contains control data indicating which sub-carriers on whichsymbols of the sub-frame have been allocated to specific LTE terminals.Thus, the PDCCH data transmitted in the control region 300 of thesub-frame shown in FIG. 3 would indicate that UE1 has been allocated thefirst block of resources 342, that UE2 has been allocated the secondblock of resources 343, and so on. The PCFICH contains control dataindicating the size of the control region (i.e. between one and threesymbols) and the PHICH contains HARQ (Hybrid Automatic Request) dataindicating whether or not previously transmitted uplink data has beensuccessfully received by the network.

In certain sub-frames, symbols in a central band 310 of the sub-frameare used for the transmission of information including the primarysynchronisation signal (PSS), the secondary synchronisation signal (SSS)and the physical broadcast channel (PBCH). This central band 310 istypically 72 sub-carriers wide (corresponding to a transmissionbandwidth of 1.08 MHz). The PSS and SSS are synchronisation signals thatonce detected allow the LTE terminal 104 to achieve framesynchronisation and determine the cell identity of the enhanced Node Btransmitting the downlink signal. The PBCH carries information about thecell, comprising a master information block (MIB) that includesparameters that the LTE terminals require to access the cell. Datatransmitted to individual LTE terminals on the physical downlink sharedchannel (PDSCH) can be transmitted in the remaining blocks of resourceelements of the sub-frame. Further explanation of these channels isprovided in the following sections.

FIG. 3 also shows a region of PDSCH containing system information andextending over a bandwidth of R₃₄₄.

The number of sub-carriers in an LTE channel can vary depending on theconfiguration of the transmission network. Typically this variation isfrom 72 sub carriers contained within a 1.4 MHz channel bandwidth to1200 sub-carriers contained within a 20 MHz channel bandwidth as shownin FIG. 3. As is known in the art, data transmitted on the PDCCH, PCFICHand PHICH is typically distributed on the sub-carriers across the entirebandwidth of the sub-frame. Therefore a conventional LTE terminal mustbe able to receive the entire bandwidth of the sub-frame in order toreceive and decode the control region.

Conventional Camp on Procedure

FIG. 4 illustrates an LTE “camp-on” process, that is the processfollowed by a terminal so that it can decode downlink transmissionswhich are sent by a base station via a downlink channel on a carrierband. Using this process, the terminal can identify the parts of thetransmissions that include system information for the cell and thusdecode configuration information for the cell.

As can be seen in FIG. 4, in a conventional LTE camp-on procedure, theterminal first synchronizes with the base station (step 400) using thePSS and SSS in the centre band 310 of the carrier as mentioned above. Ascan be seen with reference to FIG. 3 the centre band 310 has a bandwidthrange R310, where the band is at the centre of the carrier (i.e.occupying the central sub-carriers).

The terminal detects this centre band and detects the PSS and SSS whichindicate the cyclic prefix duration and the Cell ID. In LTE the PSS andSSS are only transmitted in the first and sixth sub-frames of each radioframe. Of course, in a different system, for example a non-LTE system,the band 310 may not be at the centre of the carrier band and may bewider or narrower than 72 sub-carriers or 1.08 MHz. Likewise, thesub-frames may be of a different size or sizes.

The terminal then decodes the PBCH (step 401), also carried on thecentre band 310, where the PBCH includes in particular the MasterInformation Block (MIB). The MIB indicates in particular the bandwidthR₃₂₀ of the downlink carrier, the System Frame Number (SFN), and thePHICH configuration. Using the MIB carried on the PBCH, the terminal canthen be made aware of the bandwidth R₃₂₀ of the carrier. Because theterminal also knows where the central band 310 is, it knows the exactrange R₃₂₀ of the downlink carrier.

For each sub-frame, the terminal then decodes the PCFICH which isdistributed across the entire width of carrier 320 (step 402). Asdiscussed above, an LTE downlink carrier can be up to 20 MHz wide (1200sub-carriers) and an LTE terminal therefore has to have the capabilityto receive and decode transmissions on a 20 MHz bandwidth in order todecode the PCFICH. At that stage, with a 20 MHz carrier band, theterminal operates at a much larger bandwidth (bandwidth of R₃₂₀) thanduring steps 400 and 401 (bandwidth of R₃₁₀) relating to synchronizationand PBCH decoding.

The terminal then ascertains the PHICH locations (step 403) and decodesthe PDCCH (step 404), in particular for identifying system informationtransmissions and for identifying its personal allocation grants. Theallocation grants are used by the terminal to locate system informationand to locate its data in the PDSCH. Both system information andpersonal allocations are transmitted on PDSCH and scheduled within thecarrier band 320. Steps 403 and 404 also require the terminal to operateon the entire bandwidth R320 of the carrier band.

At steps 402 to 404, the terminal decodes information contained in thecontrol region 300 of a sub-frame. As explained above, in LTE, the threecontrol channels mentioned above (PCFICH, PHICH and PDCCH) can be foundacross the control region 300 of the carrier where the control regionsextends over the range R₃₂₀ and occupies the first one, two or threeOFDM symbols of each sub-frame as discussed above. In a sub-frame,typically the control channels do not use all the resource elementswithin the control region 300, but they are scattered across the entireregion, such that a LTE terminal has to be able to simultaneouslyreceive the entire control region 300 for decoding each of the threecontrol channels.

The terminal can then decode the PDSCH (step 405) which contains systeminformation or data transmitted for this terminal.

As explained above, in an LTE sub-frame the PDSCH generally occupiesgroups of resource elements which are neither in the control region norin the resource elements occupied by PSS, SSS or PBCH. The data in theblocks of resource elements 340, 341, 342, 343 shown in FIG. 3 have asmaller bandwidth than the bandwidth of the entire carrier although todecode these blocks, a terminal first receives the PDCCH across thefrequency range R₃₂₀ and if the PDCCH indicates that a PDSCH resourceshould be decoded, once it has received the entire sub-frame, it thendecodes only the PDSCH in only the relevant frequency range indicated bythe PDCCH. So for example, UE 1 discussed above decodes the wholecontrol region 300 and then the data in the resource block 342.

Virtual Downlink Carrier

Certain classes of devices, such as MTC devices (e.g. semi-autonomous orautonomous wireless communication devices such as smart meters asdiscussed above), support communication applications that arecharacterised by the transmission of small amounts of data at relativelyinfrequent intervals and can thus be considerably less complex thanconventional LTE terminals. In many scenarios, providing low capabilityterminals such as those with a conventional high-performance LTEreceiver unit capable of receiving and processing data from an LTEdownlink frame across the full carrier bandwidth can be overly complexfor a device which only needs to communicate small amounts of data. Thismay therefore limit the practicality of a widespread deployment of lowcapability MTC type devices in an LTE network. It is preferable insteadto provide low capability terminals such as MTC devices with a simplerreceiver unit which is more proportionate with the amount of data likelyto be transmitted to the terminal. As set out below, in accordance withexamples of the present invention a “virtual carrier” is inserted in aconventional OFDM type downlink carrier (i.e. a “host carrier”). Unlikedata transmitted on a conventional OFDM type downlink carrier, datatransmitted on the virtual carrier can be received and decoded withoutneeding to process the full bandwidth of the downlink host OFDM carrier.Accordingly, data transmitted on the virtual carrier can be received anddecoded using a reduced complexity receiver unit.

FIG. 5 provides a schematic diagram illustrating an LTE downlinksub-frame which includes a virtual carrier inserted in a host carrier inaccordance with an example of the present invention.

In keeping with a conventional LTE downlink sub-frame, the first nsymbols (n is three in FIG. 5) form the control region 300 which isreserved for the transmission of downlink control data such as datatransmitted on the PDCCH. However, as can be seen from FIG. 5, outsideof the control region 300 the LTE downlink sub-frame includes a group ofresource elements below the central band 310 which form a virtualcarrier 501. As will become clear, the virtual carrier 501 is adapted sothat data transmitted on the virtual carrier 501 can be treated aslogically distinct from the data transmitted in the remaining parts ofthe host carrier and can be decoded without first decoding all thecontrol data from the control region 300. Although FIG. 5 shows thevirtual carrier occupying frequency resources below the centre band, ingeneral the virtual carrier can alternatively either occupy frequencyresources above the centre band or frequency resources including thecentre band. If the virtual carrier is configured to overlap anyresources used by the PSS, SSS or PBCH of the host carrier, or any othersignal transmitted by the host carrier that a mobile terminal operatingon the host carrier would require for correct operation and expect tofind in a known pre-determined location, the signals on the virtualcarrier can be arranged such that these aspects of the host carriersignal are maintained.

As can be seen from FIG. 5, data transmitted on the virtual carrier 501is transmitted across a limited bandwidth. This could be any suitablebandwidth providing it is smaller than that of the host carrier. In theexample shown in FIG. 5 the virtual carrier is transmitted across abandwidth comprising 12 blocks of 12 sub-carriers (i.e. 144sub-carriers) which is equivalent to a 2.16 MHz transmission bandwidth.Accordingly, a terminal receiving data transmitted on the virtualcarrier need only be equipped with a receiver capable of receiving andprocessing data transmitted over a bandwidth of 2.16 MHz. This enableslow capability terminals (for example MTC type terminals) to be providedwith simplified receiver units yet still be able to operate within anOFDM type communication network which, as explained above,conventionally requires terminals to be equipped with receivers capableof receiving and processing an OFDM signal across the entire bandwidthof the signal.

As explained above, in OFDM based mobile communication systems such asLTE, downlink data is dynamically assigned to be transmitted ondifferent sub-carriers on a sub-frame by sub-frame basis. Accordingly,in every sub-frame the network must signal which sub-carriers on whichsymbols contain data relevant to which terminals (i.e. downlink grantsignalling).

As can be seen from FIG. 3, in a conventional downlink LTE sub-framethis information is transmitted on the PDCCH during the first symbol orsymbols of the sub-frame. However, as previously explained, theinformation transmitted in the PDCCH is spread across the entirebandwidth of the sub-frame and therefore cannot be received by a mobilecommunication terminal with a simplified receiver unit capable only ofreceiving the reduced bandwidth virtual carrier.

Accordingly, as can be seen in FIG. 5, the final symbols of the virtualcarrier can be reserved as a virtual carrier control region 502 which isallocated for the transmission of control data indicating which resourceelements of the virtual carrier 501 have been allocated. In someexamples the number of symbols comprising the virtual carrier controlregion 502 is fixed for example three symbols. In other examples thevirtual carrier control region 502 can vary in size, for example betweenone and three symbols.

The virtual carrier control region can be located at any suitableposition within the virtual carrier for example in the first few symbolsof the virtual carrier. In the example of FIG. 5 this could meanpositioning the virtual carrier control region on the fourth, fifth andsixth symbols. However, fixing the position of the virtual carriercontrol region in the final symbols of the sub-frame can provide anadvantage because the position of the virtual carrier control regionneed not vary even if the number of symbols of the host carrier controlregion varies. This simplifies the processing undertaken by mobilecommunication terminals receiving data on the virtual carrier becausethere is no need for them to determine the position of the virtualcarrier control region every sub-frame as it is known that it willalways be positioned in the final symbols of the sub-frame.

In a further embodiment, the virtual carrier control symbols mayreference virtual carrier PDSCH transmissions in a separate sub-frame.

In some examples the virtual carrier may be located within the centreband 310 of the downlink sub-frame. This would minimise the reduction inhost carrier PDSCH resources caused by the insertion of a virtualcarrier since the resources occupied by the PSS/SSS and PBCH would becontained within the virtual carrier region and not the host carrierPDSCGH region. Therefore, depending on for example the expected virtualcarrier throughput, the location of a virtual carrier can beappropriately chosen to either exist inside or outside the centre bandaccording to whether the host or virtual carrier is chosen to bear theoverhead of the PSS, SSS and PBCH.

Virtual Carrier “Camp-On” Process

As explained above, before a conventional LTE terminal can begintransmitting and receiving data in a cell, it must first camp on to thecell. An adapted camp-on process must also be provided before terminalscan receive data on the virtual carrier.

FIG. 6 shows a flow diagram illustrating a camp-on process according toan example of the present invention. The virtual carrier camp-on processis explained with reference to the sub-frame shown in FIG. 5 in which avirtual carrier with a bandwidth of 144 sub-carriers is inserted in ahost carrier with a bandwidth of 1200 sub-carriers. As discussed above,a terminal having a receiver unit with an operational bandwidth of lessthan that of the host carrier cannot decode data in the control regionof sub-frames of the host carrier. However, providing the receiver unitof a terminal has an operational bandwidth of at least twelve blocks oftwelve sub-carriers (i.e. 2.16 MHz) then it can receive data transmittedon the example virtual carrier 502.

In the example of FIG. 6, the first steps 400 and 401 are the same asthe conventional camp-on process shown in FIG. 4, although a virtualcarrier terminal may extract additional information from the MIB asdescribed below. Both terminals can use the PSS/SSS and PBCH tosynchronize with the base station using the information carried on the72 sub-carrier centre band within the host carrier. However, where theconventional LTE terminals then continue with the process by performingthe PCFICH decoding step 402, which requires a receiver unit capable ofreceiving and decoding the host carrier control region 300, a terminalcamping on to the cell to receive data on the virtual carrier (referredto from now on as a “virtual carrier terminal”) performs steps 606 and607 instead.

In a further embodiment of the present invention a separatesynchronisation and PBCH functionality can be provided for the virtualcarrier device as opposed to re-using the same conventional initialcamp-on processes of steps 400 and 401 of the host carrier device.

At step 606, the virtual carrier terminal locates a virtual carrier, ifany is provided within the host carrier, using a virtualcarrier-specific step. Various possible embodiments of this step arediscussed further below. Once the virtual carrier terminal has located avirtual carrier, it can access information within the virtual carrier.For example, if the virtual carrier mirrors the conventional LTEresource allocation method, the virtual carrier terminal may then decodecontrol portions within the virtual carrier, which can for exampleindicate which resource elements within the virtual carrier have beenallocated for a specific virtual carrier terminal or for systeminformation. For example, FIG. 7 shows the blocks of resource elements350 to 352 within virtual carrier 330 that have been allocated for thesub-frame SF2. However, there is no requirement for the virtual carrierterminal to follow or mirror the conventional LTE process (e.g. steps402-404) and these steps may for example be implemented very differentlyfor a virtual carrier camp-on process.

Regardless of the virtual carrier terminal following a LTE-like step ora different type of step when performing step 607, the virtual carrierterminal can then decode the allocated resource elements at step 608 andthereby receive data transmitted by the base station. The data decodedin step 608 will include the remainder of the system informationcontaining details of the network configuration.

Even though the virtual carrier terminal does not have the bandwidthcapabilities to decode and receive downlink data if it was transmittedin the host carrier using conventional LTE, it can still access avirtual carrier within the host carrier having a limited bandwidthwhilst re-using the initial LTE steps. Step 608 may also be implementedin a LTE-like manner or in a different manner. For example, the virtualcarrier terminals may share a virtual carrier and have grants allocatedto manage the virtual carrier sharing as shown in SF2 in FIG. 7, or, inanother example, a virtual carrier terminal may have the entire virtualcarrier allocated for its own downlink transmissions, or the virtualcarrier may be entirely allocated to a virtual carrier terminal for acertain number of sub-frame only, etc.

There is therefore a degree of flexibility provided for this virtualcarrier camp-on process. There is for example given the choice to adjustthe balance between re-using or mirroring conventional LTE steps orprocesses, thereby reducing the terminal complexity and the need toimplement new elements, and adding new virtual carrier specific aspectsor implementations, thereby potentially optimizing the use ofnarrow-band virtual carriers, as LTE has been designed with thelarger-band host carriers in mind.

Downlink Virtual Carrier Detection

As discussed above, the virtual carrier terminal has to locate thevirtual carrier before it can receive and decode the virtual carriertransmissions. Several options are available for the virtual carrierpresence and location determination, which can be implemented separatelyor in combination. Some of these options are discussed below.

To facilitate the virtual carrier detection, the virtual carrierlocation information may be provided to the virtual carrier terminalsuch that it can locate the virtual carrier, if any exists, more easily.For example, such location information may comprise an indication thatone or more virtual carriers are provided within the host carrier orthat the host carrier does not currently provide any virtual carrier. Itmay also comprise an indication of the virtual carrier's bandwidth, forexample in MHz or blocks of resource elements. Alternatively, or incombination, the virtual carrier location information may comprise thevirtual carrier's centre frequency and bandwidth, thereby giving thevirtual carrier terminal the exact location and bandwidth of any activevirtual carrier. In the event that the virtual carrier is to be found ata different frequency position in each sub-frame, according for exampleto a pseudo-random hoping algorithm, the location information can forexample indicate a pseudo random parameter. Such parameters may includea starting frame and parameters used for the pseudo-random algorithm.Using these pseudo-random parameters, the virtual carrier terminal canthen know where the virtual carrier can be found for any sub-frame.

An advantageous implementation which would require little change to thevirtual carrier terminal (compared with a conventional LTE terminal) isto include this location information in the PBCH, which already carriesthe Master Information Block, or MIB in the host carrier centre band. Asshown in FIG. 8, the MIB consists of 24 bits (3 bits to indicate DLbandwidth, 8 bits to indicate the System Frame Number or SFN, and 3 bitsregarding the PHICH configuration). The MIB therefore comprises 10 sparebits that can be used to carry location information in respect of one ormore virtual carriers. For example, FIG. 9 shows an example where thePBCH includes the MIB and location information (“LI”) for pointing anyvirtual carrier terminal to a virtual carrier.

Alternatively, this Location Information can be provided for example inthe centre band, outside of the PBCH. It can for example be alwaysprovided after and adjacent to the PBCH. By providing the LocationInformation in the centre band but outside of the PBCH, the conventionalPBCH is not modified for the purpose of using virtual carriers, but avirtual carrier terminal will easily find the location information inorder to detect the virtual carrier, if any.

The virtual carrier location information, if provided, can be providedelsewhere in the host carrier, but it is advantageous to provide it inthe centre band because the virtual carrier terminal will preferentiallyconfigure its receiver to operate on the centre band and the virtualcarrier terminal then does not need to adjust its receiver settings forfinding the location information.

Depending on the amount of virtual carrier location informationprovided, the virtual carrier terminal can either adjust its receiver toreceive the virtual carrier transmissions, or it may require furtherlocation information before it can do so.

If for example, the virtual carrier terminal was provided with locationinformation indicating a virtual carrier presence and/or a virtualcarrier bandwidth but not indicating any details as to the exact virtualcarrier frequency range, or if the virtual carrier terminal was notprovided with any location information, the virtual carrier terminal canthen scan the host carrier for a virtual carrier (e.g. performing aso-called blind search process). Scanning the host carrier for a virtualcarrier can be based on different approaches, some of which will bepresented below.

According to a first approach, the virtual carrier can only be insertedin certain pre-determined locations, as illustrated for example in FIG.10 for a four-location example. The virtual carrier terminal then scansthe four locations L1-L4 for any virtual carrier. If and when thevirtual carrier terminal detects a virtual carrier, it can then“camp-on” the virtual carrier to receive downlink data. In thisapproach, the virtual carrier terminal has to know the possible virtualcarrier locations in advance, for example by reading an internal memory.Detection of a virtual carrier could be accomplished by trying to decodea known physical channel on the virtual carrier. The successful decodingof such a channel, indicated for example by a successful cyclicredundancy check (CRC) on decoded data, would indicate the successfullocation of a virtual carrier

According to a second approach, the virtual carrier may include locationsignals such that a virtual carrier terminal scanning the host carriercan detect such signals to identify the presence of a virtual carrier.Examples of possible location signals are illustrated in FIGS. 11A to11D. In the examples of FIGS. 11A to 11C, the virtual carrier regularlysends an arbitrary location signal such that a terminal scanning afrequency range where the location signal is would detect this signal.An “arbitrary” signal is meant to include any signal that does not carryany information as such, or is not meant to be interpreted, but merelyincludes a specific signal or pattern that a virtual carrier terminalcan detect. This can for example be a series of positive bits across theentire location signal, an alternation of 0 and 1 across the locationsignal, or any other suitable arbitrary signal. It is noteworthy thatthe location signal may be made of adjacent blocks of resource elementsor may be formed of non adjacent blocks. For example, it may be locatedat every other block of resource elements at the top of the virtualcarrier.

In the example of FIG. 11A, the location signal 353 extends across therange R₃₃₀ of the virtual carrier 330 and is always found at the sameposition in the virtual carrier within a sub-frame. If the virtualcarrier terminal knows where to look for a location signal in a virtualcarrier sub-frame, it can then simplify its scanning process by onlyscanning this position within a sub-frame for a location signal. FIG.11B shows a similar example where every sub-frame includes a locationsignal 354 comprising two parts: one at the top corner and one at thebottom corner of the virtual carrier sub-frame, at the end of thissub-frame. Such a location signal may become useful if for example thevirtual carrier terminal does not know the bandwidth of the virtualcarrier in advance as it can facilitate a clear detection of the top andbottom edges of the virtual carrier band.

In the example of FIG. 11C, a location signal 355 is provided in a firstsub-frame SF1, but not in a second sub-frame SF2. The location signalcan for example be provided every two sub-frames. The frequency of thelocation signals can be chosen to adjust a balance between reducingscanning time and reducing overhead. In other words, the more often thelocation signal is provided, the less long it takes a terminal to detecta virtual carrier but the more overhead there is.

In the example of FIG. 11D, a location signal is provided where thislocation signal is not an arbitrary signal as in FIGS. 11A to 11C, butis a signal that includes information for virtual carrier terminals. Thevirtual carrier terminals can detect this signal when they scan for avirtual carrier and the signal may include information in respect of,for example, the virtual carrier bandwidth or any other virtualcarrier-related information (location or non-location information). Whendetecting this signal, the virtual carrier terminal can thereby detectthe presence and location of the virtual carrier. As shown in FIG. 11D,the location signal can, like an arbitrary location signal, be found atdifferent locations within the sub-frame, and the location may vary on aper-sub-frame basis.

Dynamic Variation of Control Region Size of Host Carrier

As explained above, in LTE the number of symbols that make up thecontrol region of a downlink sub-frame varies dynamically depending onthe quantity of control data that needs to be transmitted. Typically,this variation is between one and three symbols. As will be understoodwith reference to FIG. 5, variation in the width of the host carriercontrol region will cause a corresponding variance in the number ofsymbols available for the virtual carrier. For example, as can be seenin FIG. 5, when the control region is three symbols in length and thereare 14 symbols in the sub-frame, the virtual carrier is eleven symbolslong. However, if in the next sub-frame the control region of the hostcarrier were reduced to one symbol, there would be thirteen symbolsavailable for the virtual carrier in that sub-frame.

When a virtual carrier is inserted into a LTE host carrier, mobilecommunication terminals receiving data on the virtual carrier need to beable to determine the number of symbols in the control region of eachhost carrier sub-frame to determine the number of symbols in the virtualcarrier in that sub-frame if they are to be able to use all availablesymbols that are not used by the host carrier control region.

Conventionally, the number of symbols forming the control region issignalled in the first symbol of every sub-frame in the PCFICH. However,the PCFICH is typically distributed across the entire bandwidth of thedownlink LTE sub-frame and is therefore transmitted on sub-carrierswhich virtual carrier terminals capable only of receiving the virtualcarrier cannot receive. Accordingly, in one embodiment, any symbolsacross which the control region could possibly extend are predefined asnull symbols on the virtual carrier, i.e. the length of the virtualsub-carrier is set at (m-n) symbols, where m is the total number ofsymbols in a sub-frame and n is the maximum number of symbols of thecontrol region. Thus, resource elements are never allocated for downlinkdata transmission on the virtual carrier during the first n symbols ofany given sub-frame.

Although this embodiment is simple to implement it will be spectrallyinefficient because during sub-frames when the control region of thehost carrier has fewer than the maximum number of symbols, there will beunused symbols in the virtual carrier.

In another embodiment, the number of symbols in the control region ofthe host carrier is explicitly signalled in the virtual carrier itself.Once the number of symbols in the control region of the host carrier isknown, the number of symbols in the virtual carrier can be calculated bysubtracting the total number of symbols in the sub-frame from thisnumber.

In one example an explicit indication of the host carrier control regionsize is given by certain information bits in the virtual carrier controlregion. In other words an explicit signalling message is inserted at apredefined position in the virtual carrier control region 502. Thispredefined position is known by each terminal adapted to receive data onthe virtual carrier.

In another example, the virtual carrier includes a predefined signal,the location of which indicates the number of symbols in the controlregion of the host carriers. For example, a predefined signal could betransmitted on one of three predetermined blocks of resource elements.When a terminal receives the sub-frame it scans for the predefinedsignal. If the predefined signal is found in the first block of resourceelements this indicates that the control region of the host carriercomprises one symbol; if the predefined signal is found in the secondblock of resource elements this indicates that the control region of thehost carrier comprises two symbols and if the predefined signal is foundin the third block of resource elements this indicates that the controlregion of the host carrier comprises three symbols.

In another example, the virtual carrier terminal is arranged to firstattempt to decode the virtual carrier assuming that the control regionsize of the host carrier is one symbol. If this is not successful, thevirtual carrier terminal attempts to decode the virtual carrier assumingthat the control region size of the host carrier is two and so on, untilthe virtual carrier terminal successfully decodes the virtual carrier.

Downlink Virtual Carrier Reference Signals

As is known in the art, in OFDM based transmission systems such as LTE anumber of sub-carriers in each symbol are typically reserved for thetransmission of reference signals. The reference signals are transmittedon sub-carriers distributed throughout a sub-frame across the channelbandwidth and across the OFDM symbols. The reference signals arearranged in a repeating pattern and can thus be used by a receiver,employing extrapolation and interpolation techniques to estimate thechannel function applied to the data transmitted on each sub-carrier.These reference signals are also typically used for additional purposessuch as determining metrics for received signal power indications,automatic frequency control metrics and automatic gain control metrics.In LTE the positions of the reference signal bearing sub-carriers withineach sub-frame are pre-defined and are therefore known at the receiverof each terminal.

In LTE downlink sub-frames, reference signals from each transmit antennaport are typically inserted on every sixth sub-carrier. Accordingly, ifa virtual carrier is inserted in an LTE downlink sub-frame, even if thevirtual carrier has a minimum bandwidth of one resource block (i.e.twelve sub-carriers) the virtual carrier will include at least somereference signal bearing sub-carriers.

There are sufficient reference signal bearing sub-carriers provided ineach sub-frame such that a receiver need not accurately receive everysingle reference signal to decode the data transmitted on the sub-frame.However, as will be understood the more reference signals that arereceived, the better a receiver will be able to estimate the channelresponse and hence fewer errors are typically introduced into the datadecoded from the sub-frame. Accordingly, in order to preservecompatibility with LTE communication terminals receiving data on thehost carrier, in some examples of the present invention, the sub-carrierpositions that would contain reference signals in a conventional LTEsub-frame are retained in the virtual carrier.

As will be understood, in accordance with examples of the presentinvention, terminals arranged to receive only the virtual carrierreceive a reduced number of sub-carriers compared to conventional LTEterminals which receive each sub-frame across the entire bandwidth ofthe sub-frame. As a result, the reduced capability terminals receivefewer reference signals over a narrower range of frequencies which mayresult in a less accurate channel estimation being generated.

In some examples a simplified virtual carrier terminal may have a lowermobility which requires fewer reference symbols to support channelestimation. However, in some examples of the present invention thedownlink virtual carrier includes additional reference signal bearingsub-carriers to enhance the accuracy of the channel estimation that thereduced capability terminals can generate.

In some examples the positions of the additional reference bearingsub-carriers are such that they are systematically interspersed withrespect to the positions of the conventional reference signal bearingsub-carriers thereby increasing the sampling frequency of the channelestimation when combined with the reference signals from the existingreference signal bearing sub-carriers. This allows an improved channelestimation of the channel to be generated by the reduced capabilityterminals across the bandwidth of the virtual carrier. In otherexamples, the positions of the additional reference bearing sub-carriersare such that they are systematically placed at the edge of thebandwidth of the virtual carrier thereby increasing the interpolationaccuracy of the virtual carrier channel estimates.

Alternative Virtual Carrier Arrangements

So far examples of the invention have been described generally in termsof a host carrier in which a single virtual carrier has been inserted asshown for example in FIG. 5. However, in some examples a host carriermay include more than one virtual carrier as shown for example in FIG.12. FIG. 12 shows an example in which two virtual carriers VC1 (330) andVC2 (331) are provided within a host carrier 320. In this example, thetwo virtual carriers change location within the host carrier bandaccording to a pseudo-random algorithm. However, in other examples, oneor both of the two virtual carriers may always be found in the samefrequency range within the host carrier frequency range and/or maychange position according to a different mechanism. In LTE, the numberof virtual carriers within a host carrier is only limited by the size ofthe host carrier. However, too many virtual carriers within the hostcarrier may unduly limit the bandwidth available for transmitting datato conventional LTE terminals and an operator may therefore decide on anumber of virtual carrier within a host carrier according to, forexample, a ratio of conventional LTE users/virtual carrier users.

In some examples the number of active virtual carriers can bedynamically adjusted such that it fits the current needs of conventionalLTE terminals and virtual carrier terminals. For example, if no virtualcarrier terminal is connected or if their access is to be intentionallylimited, the network can arrange to begin scheduling the transmission ofdata to LTE terminals within the sub-carriers previously reserved forthe virtual carrier. This process can be reversed if the number ofactive virtual carrier terminals begins to increase. In some examplesthe number of virtual carriers provided may be increased in response toan increase in the presence of virtual carrier terminals. For example ifthe number of virtual terminals present in a network or area of anetwork exceeds a threshold value, an additional virtual carrier isinserted in the host carrier. The network elements and/or networkoperator can thus activate or deactivate the virtual carriers wheneverappropriate.

The virtual carrier shown for example in FIG. 5 is 144 sub-carriers inbandwidth. However, in other examples a virtual carrier may be of anysize between twelve sub-carriers to 1188 sub-carriers (for a carrierwith a 1200 sub-carrier transmission bandwidth). Because in LTE thecentre band has a bandwidth of 72 sub-carriers, a virtual carrierterminal in an LTE environment preferentially has a receiver bandwidthof at least 72 sub-carriers (1.08 MHz) such that it can decode thecentre band 310, therefore a 72 sub-carrier virtual carrier may providea convenient implementation option. With a virtual carrier comprising 72sub-carriers, the virtual carrier terminal does not have to adjust thereceiver's bandwidth for camping on the virtual carrier which maytherefore reduce complexity of performing the camp-on process, but thereis no requirement to have the same bandwidth for the virtual carrier asfor the centre band and, as explained above, a virtual carrier based onLTE can be of any size between 12 to 1188 sub-carriers. For example, insome systems, a virtual carrier having a bandwidth of less than 72sub-carriers may be considered as a waste of the virtual carrierterminal's receiver resources, but from another point of view, it may beconsidered as reducing the impact of the virtual carrier on the hostcarrier by increasing the bandwidth available to conventional LTEterminals. The bandwidth of a virtual carrier can therefore be adjustedto achieve the desired balance between complexity, resource utilization,host carrier performance and requirements for virtual carrier terminals.

Uplink Transmission Frame

So far, the virtual carrier has been discussed with reference to thedownlink, however in some examples a virtual carrier can also beinserted in the uplink.

In mobile communications systems such as LTE, the frame structure andsub-carrier spacing employed in the uplink correspond to that used inthe downlink (as shown for example in FIG. 2). In frequency divisionduplex (FDD) networks both the uplink and downlink are active in allsub-frames, whereas in time division duplex (TDD) networks sub-framescan either be assigned to the uplink, to the downlink, or furthersub-divided into uplink and downlink portions.

In order to initiate a connection to a network, conventional LTEterminals make a random access request on the physical random accesschannel (PRACH). The PRACH is located in predetermined blocks ofresource elements in the uplink frame, the positions of which aresignaled to the LTE terminals in the system information signaled on thedownlink.

Additionally, when there is pending uplink data to be transmitted froman LTE terminal and the terminal does not already have any uplinkresources allocated to it, it can transmit a random access request PRACHto the base station. A decision is then made at the base station as towhich if any uplink blocks of resource elements are to be allocated tothe mobile terminal that has made the request. Uplink resource blockallocations are then signaled to the LTE terminal on the physicaldownlink control channel (PDCCH) transmitted in the control region ofthe downlink sub-frame.

In LTE, transmissions from each mobile terminal are constrained tooccupy a set of contiguous resource blocks. For the physical uplinkshared channel (PUSCH) the uplink resource allocation grant receivedfrom the base station will indicate which set of resource blocks to usefor that transmission, where these resource blocks could be locatedanywhere within the channel bandwidth.

The first resources used by the LTE physical uplink control channel(PUCCH) are located at both the upper and lower edge of the channel,where each PUCCH transmission occupies one resource block. In the firsthalf of a sub-frame this resource block is located at one channel edge,and in the second half of a sub-frame this resource block is located atthe opposite channel edge. As more PUCCH resources are required,additional resource blocks are assigned in a sequential manner, movinginward from the channel edges. Since PUCCH signals are code divisionmultiplexed, an LTE uplink can accommodate multiple PUCCH transmissionsin the same resource block.

Virtual Uplink Carrier

In accordance with embodiments of the present invention, the virtualcarrier terminals described above can also be provided with a reducedcapability transmitter for transmitting uplink data. The virtual carrierterminals are arranged to transmit data across a reduced bandwidth. Theprovision of a reduced capability transmitter unit providescorresponding advantages to those achieved by providing a reducedcapability receiver unit with, for example, classes of devices that aremanufactured with a reduced capability for use with, for example, MTCtype applications.

In correspondence with the downlink virtual carrier, the virtual carrierterminals transmit uplink data across a reduced range of sub-carrierswithin a host carrier that has a greater bandwidth than that of thereduced bandwidth virtual carrier. This is shown in FIG. 13a . As can beseen from FIG. 13a , a group of sub-carriers in an uplink sub-frame forma virtual carrier 1301 within a host carrier 1302. Accordingly, thereduced bandwidth across which the virtual carrier terminals transmituplink data can be considered a virtual uplink carrier.

In order to implement the virtual uplink carrier, the base stationscheduler serving a virtual carrier ensures that all uplink resourceelements granted to virtual carrier terminals are sub-carriers that fallwithin the reduced bandwidth range of the reduced capability transmitterunits of the virtual carrier terminals. Correspondingly, the basestation scheduler serving the host carrier typically ensures that alluplink resource elements granted to host carrier terminals aresub-carriers that fall outside the set of sub-carriers occupied by thevirtual carrier terminals. However, if the schedulers for the virtualcarrier and the host carrier are implemented jointly, or have means toshare information, then the scheduler of the host carrier can assignresource elements from within the virtual carrier region to mobileterminals on the host carrier during sub-frames when the virtual carrierscheduler indicates that some or all of the virtual carrier resourceswill not be used by mobile terminals on the virtual carrier.

If a virtual carrier uplink incorporates a physical channel that followsa similar structure and method of operation to the LTE PUCCH, whereresources for that physical channel are expected to be at the channeledges, for virtual carrier terminals these resources wouldpreferentially be at the edges of the virtual carrier and not at theedges of the host carrier. This is advantageous since it would ensurethat virtual carrier uplink transmissions remain within the reducedvirtual carrier bandwidth.

Virtual Uplink Carrier Random Access

In accordance with conventional LTE techniques, it cannot be guaranteedthat the PRACH will be within the sub-carriers allocated to the virtualcarrier. In some embodiments therefore, the base station provides asecondary PRACH within the virtual uplink carrier, the location of whichcan be signaled to the virtual carrier terminals via system informationon the virtual carrier. This is shown for example in FIG. 13b in which aPRACH 1303 is located within the virtual carrier 1301. Thus, the virtualcarrier terminals send PRACH requests on the virtual carrier PRACHwithin the virtual uplink carrier. The position of the PRACH can besignaled to the virtual carrier terminals in a virtual carrier downlinksignaling channel, for example in system information on the virtualcarrier.

However, in other examples, the virtual carrier PRACH 1303 is situatedoutside of the virtual carrier as shown for example in FIG. 13c . Thisleaves more room within the virtual uplink carrier for the transmissionof data by the virtual carrier terminals. The position of the virtualcarrier PRACH is signaled to the virtual carrier terminals as before butin order to transmit a random access request, the virtual carrierterminals re-tune their transmitter units to the virtual carrier PRACHfrequency because it is outside of the virtual carrier. The transmitterunits are then re-tuned to the virtual carrier frequency when uplinkresource elements have been allocated.

In some examples where the virtual carrier terminals are capable oftransmitting on a PRACH outside of the virtual carrier, the position ofthe host carrier PRACH can be signaled to the virtual carrier terminals.The virtual carrier terminals can then simply use the conventional hostcarrier PRACH resource to send random access requests. This approach isadvantageous as fewer PRACH resources have to be allocated.

However, if the base station is receiving random access requests fromboth conventional LTE terminals and virtual carrier terminals on thesame PRACH resource, it is necessary that the base station is providedwith a mechanism for distinguishing between random access requests fromconventional LTE terminals and random access requests from virtualcarrier terminals.

Therefore, in some examples a time division allocation is implemented atthe base station whereby, for example, over a first set of sub-framesthe PRACH allocation is available to the virtual carrier terminals andover a second set of sub-frames the PRACH allocation is available toconventional LTE terminals. Accordingly, the base station can determinethat random access requests received during the first set of sub-framesoriginate from virtual carrier terminals and random access requestsreceived during the second set of sub-frames originate from conventionalLTE terminals.

In other examples, no mechanism is provided to prevent both virtualcarrier terminals and conventional LTE terminals transmitting randomaccess requests at the same time. However, the random access preamblesthat are conventionally used to transmit a random access request aredivided into two groups. The first group is used exclusively by virtualcarrier terminals and the second group is used exclusively byconventional LTE terminals. Accordingly, the base station can determinewhether a random request originated from a conventional LTE terminal ora virtual carrier terminal simply by ascertaining to what group therandom access preamble belongs.

Example Architecture

FIG. 14 provides a schematic diagram showing part of an adapted LTEmobile telecommunication system arranged in accordance with an exampleof the present invention. The system includes an adapted enhanced Node B(eNB) 1401 connected to a core network 1408 which communicates data to aplurality of conventional LTE terminals 1402 and reduced capabilityterminals 1403 within a coverage area (i.e. cell) 1404. Each of thereduced capability terminals 1403 has a transceiver unit 1405 whichincludes a receiver unit capable of receiving data across a reducedbandwidth and a transmitter unit capable of transmitting data across areduced bandwidth when compared with the capabilities of the transceiverunits 1406 included in the conventional LTE terminals 1402.

The adapted eNB 1401 is arranged to transmit downlink data using asub-frame structure that includes a virtual carrier as described withreference to FIG. 5 and to receive uplink data using a sub-framestructure as described with reference to FIG. 13b or 13 c. The reducedcapability terminals 1403 are thus able to receive and transmit datausing the uplink and downlink virtual carriers as described above.

As has been explained above, because the reduced complexity terminals1403 receive and transmit data across a reduced bandwidth on the uplinkand downlink virtual carriers, the complexity, power consumption andcost of the transceiver unit 1405 needed to receive and decode downlinkdata and to encode and transmit uplink data is reduced compared to thetransceiver unit 1406 provided in the conventional LTE terminals.

When receiving downlink data from the core network 1408 to betransmitted to one of the terminals within the cell 1404, the adaptedeNB 1401 is arranged to determine if the data is bound for aconventional LTE terminal 1402 or a reduced capability terminal 1403.This can be achieved using any suitable technique. For example, databound for a reduced capability terminal 1403 may include a virtualcarrier flag indicating that the data must be transmitted on thedownlink virtual carrier. If the adapted eNB 1401 detects that downlinkdata is to be transmitted to a reduced capability terminal 1403, anadapted scheduling unit 1409 included in the adapted eNB 1401 ensuresthat the downlink data is transmitted to the reduced capability terminalin question on the downlink virtual. In another example the network isarranged so that the virtual carrier is logically independent of theeNB. More particularly the virtual carrier is arranged to appear to thecore network as a distinct cell. From the perspective of the corenetwork it is not known that the virtual carrier is physicallyco-located with, or has any interaction with, the host carrier of thecell. Packets are routed to/from the virtual carrier just as they wouldbe for any normal cell.

In another example, packet inspection is performed at a suitable pointwithin the network to route traffic to or from the appropriate carrier(i.e. the host carrier or the virtual carrier).

In yet another example, data from the core network to the eNB iscommunicated on a specific logical connection for a specific mobileterminal. The eNB is provided with information indicating which logicalconnection is associated with which mobile terminal. Information is alsoprovided at the eNB indicating which mobile terminals are virtualcarrier terminals and which are conventional LTE terminals. Thisinformation could be derived from the fact that a virtual carrierterminal would initially have connected using virtual carrier resources.In other examples virtual carrier terminals are arranged to indicatetheir capability to the eNB during the connection procedure. Accordinglythe eNB can map data from the core network to a specific mobile terminalbased on whether the mobile terminal is a virtual carrier terminal or anLTE terminal.

When scheduling resources for the transmission of uplink data, theadapted eNB 1401 is arranged to determine if the terminal to bescheduled resources is a reduced capability terminal 1403 or aconventional LTE terminal 1402. In some examples this is achieved byanalysing the random access request transmitted on the PRACH using thetechniques to distinguish between a virtual carrier random accessrequest and a conventional random access request as described above. Inany case, when it has been determined at the adapted eNB 1401 that arandom access request has been made by a reduced capability terminal1402, the adapted scheduler 1409 is arranged to ensure that any grantsof uplink resource elements are within the virtual uplink carrier.

In some examples, the virtual carrier inserted within the host carriercan be used to provide a logically distinct “network within a network”.In other words data being transmitted via the virtual carrier can betreated as logically and physically distinct from the data transmittedby the host carrier network. The virtual carrier can therefore be usedto implement a so-called dedicated messaging network (DMN) which is“laid over” a conventional network and used to communicate messagingdata to DMN devices (i.e. virtual carrier terminals).

As will be appreciated from the above descriptions, embodiments of thepresent invention can include the following examples:

A method of allocating transmission resources in an OFDM wirelesstelecommunication system arranged to communicate data using a pluralityof OFDM sub-carriers, the method comprising:

allocating transmission resources provided by a first group of theplurality of OFDM sub-carriers within a first frequency band toterminals of a first type;

allocating transmission resources provided by a second group of theplurality of OFDM sub-carriers to terminals of a second type within asecond frequency band, the second group being smaller than the firstgroup and the second frequency band being selected from within the firstfrequency band;

transmitting control information comprising resource allocationinformation for terminals of the first type over a first bandwidthcorresponding to the combined first and second groups of OFDMsub-carriers; and

transmitting control information comprising resource allocationinformation for terminals of the second type over a second bandwidthcorresponding to the second group of OFDM sub-carriers.

An OFDM wireless telecommunication system arranged to communicate datato and from a plurality of mobile terminals over a plurality of OFDMsub-carriers, the system comprising

scheduling means arranged to allocate transmission resources provided bya first group of the plurality of OFDM sub-carriers within a firstfrequency band to mobile terminals of a first type and to allocatetransmission resources provided by a second group of the plurality ofOFDM sub-carriers within a second frequency band to terminals of asecond type, the second group being smaller than the first group and thesecond frequency band being selected from within the first frequencyband, and

transmission means arranged to transmit control information comprisingresource allocation information for terminals of the first type over afirst bandwidth corresponding to the combined first and second groups ofOFDM sub-carriers and to transmit control information comprisingresource allocation information for terminals of the second type over asecond bandwidth corresponding to the second group of OFDM sub-carriers.

A mobile terminal comprising a receiver unit for receiving datatransmitted from a base station via a plurality of OFDM sub-carriers ona radio downlink and a transmitter for transmitting data to the basestation via a plurality of OFDM sub-carriers on a radio uplink, the basestation being arranged to transmit data to mobile terminals of a firsttype on a first group of a plurality of OFDM sub-carriers within a firstfrequency band and to transmit data to mobile terminals of a second typeto which the mobile terminal belongs on a second group of the pluralityof OFDM sub-carriers within a second frequency band, the second groupbeing smaller than the first group and the second frequency band beingselected from within the first frequency band, the base station beingarranged to transmit control information comprising resource allocationinformation for terminals of the first type over a first bandwidthcorresponding to the combined first and second groups of OFDMsub-carriers and to transmit control information comprising resourceallocation information for terminals of the second type over a secondbandwidth corresponding to the second group of OFDM sub-carriers,wherein the receiver unit of the mobile terminal is limited to receivedata on the radio downlink over the second frequency band.

A network element for use in a mobile communications system, the networkelement being operable to:

provide a wireless access interface for communicating data to and/orfrom the mobile communications devices, the wireless access interfaceproviding on a downlink a host carrier, the host carrier providing aplurality of resource elements across a first frequency range,

transmit data for a first group of mobile communications devices,wherein the data is distributed within the plurality of resourceelements across the first frequency range;

provide a virtual carrier via the wireless access interface, the virtualcarrier providing one or more resource elements within a secondfrequency range which is within and smaller than the first frequencyrange; and

transmit data for a second group of mobile communications devices viathe virtual carrier.

A method of using a network element for communicating data to and/orfrom mobile communications devices in a mobile communications system,the method comprising:

providing a wireless access interface for communicating data to and/orfrom the mobile communications devices, the wireless access interfaceproviding on a downlink a host carrier, the host carrier providing aplurality of resource elements across a first frequency range,

transmitting data for a first group of mobile communications devices,wherein the data is distributed within the plurality of resourceelements across the first frequency range;

providing a virtual carrier via the wireless access interface, thevirtual carrier providing one or more resource elements within a secondfrequency range which is within and smaller than the first frequencyrange; and

transmitting data for a second group of mobile communications devicesvia the at least one virtual carrier.

A base station for communicating data to and from a plurality of mobileterminals over a plurality of OFDM sub-carriers within a coverage areaprovided by the base station, the base station arranged to allocatetransmission resources provided by a first group of the plurality ofOFDM sub-carriers within a first frequency band to mobile terminals of afirst type and to allocate transmission resources provided by a secondgroup of the plurality of OFDM sub-carriers within a second frequencyband to terminals of a second type, the second group being smaller thanthe first group and the second frequency band being selected from withinthe first frequency band, and

to transmit control information comprising resource allocationinformation for terminals of the first type over a first bandwidthcorresponding to the combined first and second groups of OFDMsub-carriers and to transmit control information comprising resourceallocation information for terminals of the second type over a secondbandwidth corresponding to the second group of OFDM sub-carriers.

A mobile communications system for communicating data to and/or frommobile communications devices, the mobile communications systemcomprising:

one or more base stations, each of which includes a transmitter and areceiver operable to provide a wireless access interface forcommunicating data to and/or from the mobile communications devices, thewireless access interface providing on a downlink a host carrier, thehost carrier providing a plurality of resource elements across a firstfrequency range for communicating data, and

a first and second mobile communications devices,

wherein:

the first mobile communications device is operable to receive downlinkcommunications via the host carrier,

the wireless access interface provided by the one or more base stationsis arranged to provide a virtual carrier, the virtual carrier providingone or more resource elements within a second frequency range which iswithin and smaller than the first frequency range, and

the second mobile communications device is operable, upon detection ofthe virtual carrier, to receive downlink communications via the virtualcarrier.

A method of communicating data to and/or from mobile communicationsdevices in a mobile communications system, the method comprising:

providing a wireless access interface for communicating data to and/orfrom the mobile communications devices, the wireless access interfaceproviding on a downlink a host carrier, the host carrier providing aplurality of resource elements across a first frequency range,

transmitting data for a first group of mobile communications devices,wherein the data is distributed within the plurality of resourceelements across the first frequency range;

a first mobile communications device in the first group of mobilecommunications devices receiving downlink communications via the hostcarrier;

providing a virtual carrier via the wireless access interface, thevirtual carrier providing one or more resource elements within a secondfrequency range which is within and smaller than the first frequencyrange; and

transmitting data for a second group of mobile communications devicesvia the at least one virtual carrier.

a second mobile communications device in the second group of mobilecommunications devices detecting the virtual carrier; and

the second mobile communications device receiving downlinkcommunications via the virtual carrier.

Various modifications can be made to examples of the present invention.Embodiments of the present invention have been defined largely in teemsof reduced capability terminals transmitting data via a virtual carrierinserted in a conventional LTE based host carrier. However, it will beunderstood that any suitable device can transmit and receive data usingthe described virtual carriers for example devices which have the samecapability as a conventional LTE type terminal or devices which haveenhanced capabilities.

Furthermore, it will be understood that the general principle ofinserting a virtual carrier on a subset of uplink or downlink resourcescan be applied to any suitable mobile telecommunication technology andneed not be restricted to systems employing an LTE based radiointerface.

The invention claimed is:
 1. A base station that communicates with aplurality of terminals over a plurality of orthogonal frequency-divisionmultiplexing (OFDM) sub-carriers within a coverage area provided by thebase station, the base station comprising: circuitry configured toallocate transmission resources provided by a first group of theplurality of OFDM sub-carriers within first frequency resources toterminals of a first type; allocate transmission resources provided by asecond group of the plurality of OFDM sub-carriers within secondfrequency resources to terminals of a second type, the second groupbeing smaller than the first group and the second frequency resourcesbeing a component within the first frequency resources; transmit firstcontrol information with one or more first OFDM symbols over a firstbandwidth corresponding to a combination of the first and second groupsof the plurality of OFDM sub-carriers, the first control informationcomprising first resource allocation information for the terminals ofthe first type; transmit second control information with one or moresecond OFDM symbols over a second bandwidth corresponding to the secondgroup of OFDM sub-carriers, the second control information comprisingsecond resource allocation information for the terminals of the secondtype; and transmit first data with one or more third OFDM symbols overthe second bandwidth, wherein the OFDM sub-carriers are transmittedusing a sub-frame structure, the one or more first OFDM symbols aredifferent from the one or more second OFDM symbols and the one or morethird OFDM symbols, and the one or more second OFDM symbols aredifferent from the one or more third OFDM symbols.
 2. The base stationaccording to claim 1, wherein the circuitry is configured to transmitthe second control information in a first sub-frame which relates toallocation of resources in a subsequent sub-frame.
 3. The base stationaccording to claim 1, wherein the circuitry is configured to transmitthe second control information in a first sub-frame which relates toallocation of resources in the same sub-frame.
 4. The base stationaccording to claim 1, wherein the second group of the plurality of theOFDM sub-carriers form a virtual carrier inserted in the first group ofthe plurality of the OFDM sub-carriers, the first group of the pluralityof the OFDM sub-carriers form a host carrier, and the circuitry isconfigured to transmit the first data to the terminals of the secondtype on the virtual carrier and to transmit second data to the terminalsof the first type on the host carrier.
 5. The base station according toclaim 4, wherein the circuitry is configured to insert one or moreadditional virtual carriers in the host carrier.
 6. The base stationaccording to claim 4, wherein the circuitry is configured to transmitreference signals for channel estimation, for use by both the terminalsof the first type and terminals of the second type, in the virtualcarrier.
 7. The base station according to claim 6, wherein the circuitryis configured to transmit additional reference signals within thevirtual carrier for channel estimation, for use by terminals of thesecond type, in the virtual carrier.
 8. The base station according toclaim 4, wherein a number of the one or more first OFDM symbols variesfrom sub-frame to sub-frame.
 9. The base station according to claim 4,wherein the circuitry is configured to transmit the virtual carrier overa final m-n symbols of each sub-frame, where n is a number of the one ormore first OFDM symbols and m corresponds to a number of symbols in thesub-frame, and signal to terminals of the second type an indication of nfor each given sub-frame to enable the terminals of the second type todetermine a length of the virtual carrier for each given sub-frame. 10.The base station according to claim 4, wherein the one or more secondOFDM symbols are the last OFDM symbols of each sub-frame.
 11. The basestation according to claim 1, wherein the base station is configured inaccordance with 3GPP Long Term Evolution (LTE) specifications.
 12. Thebase station according to claim 1, wherein the one or more second OFDMsymbols follow the one or more third OFDM symbols in each sub-frame. 13.The base station according to claim 1, wherein the one or more thirdOFDM symbols follow the one or more first OFDM symbols in eachsub-frame.
 14. The base station according to claim 1, wherein thecircuitry transmits second data to the terminals of the first type withthe one or more second OFDM symbols and/or the one or more third OFDMsymbols over the first bandwidth.
 15. An apparatus that communicateswith a plurality of terminals over a plurality of orthogonalfrequency-division multiplexing (OFDM) sub-carriers within a coveragearea, the apparatus comprising: scheduling circuitry that allocatestransmission resources provided by a first group of the plurality ofOFDM sub-carriers within first frequency resources to terminals of afirst type; and allocates transmission resources provided by a secondgroup of the plurality of OFDM sub-carriers within second frequencyresources to terminals of a second type, the second group being smallerthan the first group and the second frequency resources being acomponent of the first frequency resources; and transmission circuitrythat transmits first control information with one or more first OFDMsymbols over a first bandwidth corresponding to a combination of thefirst and second groups of the plurality of OFDM sub-carriers, the firstcontrol information comprising first resource allocation information forthe terminals of the first type; transmits second control informationwith one or more second OFDM symbols over a second bandwidthcorresponding to the second group of OFDM sub-carriers, the secondcontrol information comprising second resource allocation informationfor the terminals of the second type; and transmits first data with oneor more third OFDM symbols over the second bandwidth, wherein the OFDMsub-carriers are transmitted using a sub-frame structure, the one ormore first OFDM symbols are different from the one or more second OFDMsymbols and the one or more third OFDM symbols, and the one or moresecond OFDM symbols are different from the one or more third OFDMsymbols.
 16. The apparatus according to claim 15, wherein the secondgroup of the plurality of the OFDM sub-carriers form a virtual carrierinserted in the first group of the plurality of the OFDM sub-carriers,the first group of the plurality of the OFDM sub-carriers form a hostcarrier, and the transmission circuitry is configured to transmit thefirst data to the terminals of the second type on the virtual carrierand to transmit second data to the terminals of the first type on thehost carrier.